LED arrangement

ABSTRACT

The invention concerns an LED arrangement with at least one LED chip comprising a radiation decoupling surface through which the bulk of the electromagnetic radiation generated in the LED chip is decoupled. Arranged on the radiation decoupling surface is at least one phosphor layer for converting the electromagnetic radiation generated in the LED chip. A housing envelops portions of the LED chip and the phosphor layer.

CROSS REFERENCE TO RELATED APPLICATION

Pursuant to 35 U.S.C. § 120, this application is a divisionalapplication of U.S. application Ser. No. 11/119,403, filed Apr. 29, 2005now U.S. Pat. No. 7,208,769, which claims the benefit of GermanApplication No. 102004021233.3, filed Apr. 30, 2004. The contents of theprior applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

This invention relates to an LED (light-emitting diode) arrangement anda method of fabricating such an LED arrangement.

BACKGROUND

The document U.S. Pat. No. 6,657,379 B2 concerns an LED arrangementcomprising at least one LED that emits light in a wavelength range of300 to 485 nm. The emitted light is partially or completely convertedinto preferably longer-range radiation by phosphors that are exposed tothe primary radiation of the LED. The LED chip is disposed in a recessin a housing. The walls of the housing are implemented as reflective,and the recess is filled with a compound that completely envelops theLED chip. The phosphor particles for converting the light given off bythe LED chip are intermixed with this compound.

The document DE 100 204 65 A1 describes a radiation-emittingsemiconductor component comprising a luminescence conversion element.Here, an LED chip is disposed in a recess in a base body. Inside therecess in the housing, a dish-like region is hollowed out around thesemiconductor body to contain a luminescence conversion element thatenvelops the diode chip. The inner lateral surfaces of the dish-likeregion are implemented as reflective. The rest of the recess is filledwith a transparent compound.

It is an object of the instant invention to specify an LED arrangementthat can be fabricated in a particularly cost-effective manner. It isfurther an object of the invention to specify a method of fabricatingsuch an LED arrangement.

This object is achieved by means of an LED arrangement as set forthbelow.

SUMMARY

An LED arrangement is specified that comprises at least one LED chip.The LED chip possesses a radiation decoupling surface. The radiationdecoupling surface is, for example, formed by one of the surfaces of theLED chip. The bulk of the electromagnetic radiation generated in the LEDchip is decoupled via this radiation decoupling surface.

Arranged on the radiation decoupling surface is at least one phosphorlayer. Said phosphor layer preferably contains a matrix into which aluminescence conversion material is mixed. The term “luminescenceconversion material” herein is to be understood as a material comprisingconstituents by means of which the electromagnetic radiation decoupledthrough the radiation decoupling surface is converted intoelectromagnetic radiation of a modified wavelength. Hence, the phosphorlayer serves to convert the electromagnetic radiation generated by theLED chip into electromagnetic radiation of another wavelength. Thephosphor layer is particularly preferably between 10 and 50 μm thick.

Furthermore, the LED chip is embedded in a housing. That is, the LEDchip is preferably enveloped form-fittingly by a housing, the surface ofthe phosphor layer opposite the radiation decoupling surface of the LEDchip being at least partially not covered by the housing.

In a preferred embodiment of the LED arrangement, a first contact layeris applied sidewardly to the radiation decoupling surface. That is, thecontact layer is for example disposed in a corner of the radiationdecoupling surface. In this case, the first contact layer covers only asmall portion of the radiation decoupling surface and serves to effectelectrical contacting of the LED chip. The contact layer preferablycontains a metal. The sideward arrangement of the contact layeradvantageously achieves the effect that by far the majority of theradiating surface of the contact layer is uncovered and theelectromagnetic radiation generated in the LED chip can therefore bedecoupled through the radiation decoupling surface unimpeded. Thecontact layer preferably forms the cathode of the LED chip.

Particularly preferably, the contact layer forms a bonding pad forcontacting the LED chip by means of, for example, wire bonding. Thismeans that the LED chip is preferably contacted electrically via thecontact layer by means of a lead. Both the contact layer and the leadare preferably enveloped by the housing. This also advantageouslyincreases the mechanical stability of the connection between the bondingpad and the lead. That is, the contact layer is also covered, at leastin places, with housing material.

In a further preferred embodiment of the LED arrangement, theradiation-generating layer of the LED chip is arranged between areflecting layer and the radiation decoupling surface. This reflectinglayer has proven particularly advantageous both for the radiationintensity of the LED chip and for the conversion of the emitted light.For instance, radiation generated in the radiation-generating layer thatis, for example, reflected back into the LED chip by a particle of theluminescence conversion material is re-reflected by the mirror face ofthe reflecting layer and thus can again be decoupled through theradiation decoupling surface. The light output of such an LEDarrangement is therefore particularly high.

The radiation-generating layer is particularly preferably implemented asan epitaxial layer sequence, and comprises, for example, amulti-quantum-well structure for generating the electromagneticradiation.

In a particularly preferred embodiment of the LED arrangement, the LEDchip is a thin-film LED chip. Such a thin-film LED chip is distinguishedin particular by the following features:

-   -   Applied to or formed on a first main surface of the        radiation-generating epitaxial layer sequence of the LED chip,        which surface faces a carrier element, is a reflecting layer        that reflects back at least some of the electromagnetic        radiation generated in the radiation-generating layer.    -   The radiation-generating layer has a thickness in the region of        20 μm or less, particularly in the region of 10 μm.    -   The radiation-generating layer comprises at least one        semiconductor layer that has at least one surface with an        intermixed structure, which in the ideal case brings about a        nearly ergodic distribution of the light in the        radiation-generating layer, i.e., said layer has a scattering        behavior that is as ergodic as possible.

A basic principle of a thin-film LED chip is described, for example, inI. Schnitzer et al., Appl. Phys. Lett. 63 (16, 18), October 1993,2174-2176, whose disclosure content in that regard is incorporatedherein by reference.

A thin-film LED is, as a good approximation, a Lambertian surfaceradiator, and is therefore particularly well suited for use in asearchlight or headlight.

In a particularly preferred embodiment of the LED diode arrangement, thehousing in which the LED chip is embedded is implemented asradiation-absorbing. “Radiation-absorbing” in this context means, forexample, that the housing is implemented as absorptive of either theelectromagnetic radiation emitted by the LED chip or the environmentallight. Such a housing preferably contains black mold compound. Said moldcompound can, for example, be blackened with soot. The blackening of themold compound advantageously creates good contrast between the lightexit surface of the LED arrangement and the surrounding housing.Improving the housing by making it radiation-absorbing has provenparticularly advantageous, since it permits a defined light output. Theelectromagnetic radiation generated in the LED chip can therefore leavethe LED arrangement substantially only at the locations intended forthat purpose, i.e., through the radiation decoupling surface and thusthrough the phosphor layer. Thus, since nearly all the radiationgenerated in the LED chip leaves the arrangement through the phosphorlayer, the radiation-absorbing housing also contributes to particularlyeffective conversion of the emitted radiation.

In a further preferred embodiment, a covering body is disposed on thephosphor layer. The covering body is advantageously formed substantiallyof a material whose thermal expansion coefficient is the same or verynearly the same as the thermal expansion coefficient of the phosphorlayer. The covering body is preferably formed in the same materialsystem as the matrix of the phosphor layer. This advantageously reducesmechanical stress on the LED arrangement during the operation of the LEDchip.

The covering body is particularly preferably implemented as an opticalelement. The optical element advantageously affords the possibility ofadapting the radiation characteristic of the LED arrangement to the areaof application of said arrangement. In particular, the optical elementcan in this case be implemented as a refractive or diffractive lens.

In a particularly preferred embodiment of the LED arrangement, the LEDchip is disposed on a carrier. The LED chip is preferably bonded to thecarrier. The carrier preferably serves to effect both mechanicalattachment and electrical contacting of the LED chip. To this end, thecarrier is preferably structured appropriately. That is, the carriercomprises, for example, electrical leads and connection sites forcontacting the chip.

In a particularly preferred embodiment of the LED arrangement, said LEDarrangement comprises a multiplicity of LED chips. The arrangement ofthe LED chips with respect to one another is preferably adapted to theuse requirements of the LED arrangement. For example, the geometricalarrangement of the LED chips can be adapted to the location where theLED arrangement is to be used. The shape of the carrier and of thehousing can also be suitably adapted to such use requirements.

The LED arrangement as a whole makes use, inter alia, of the idea thatthe combination of an LED chip comprising a highly reflective mirrorlayer and a thin phosphor layer disposed on the radiation decouplingsurface permits very efficient conversion of the emitted radiation,since the bulk of the electromagnetic radiation generated in the LEDchip is decoupled through the conversion layer. Advantageously, thedefined overmolding of the LED chip with a radiation-absorbing moldcompound as a housing results in good contrast values and defineddecoupling of the electromagnetic radiation through the radiationdecoupling surface and thus the phosphor layer.

Further specified is a method of fabricating an LED arrangement,comprising the following steps:

-   -   a) preparing a carrier,    -   b) attaching an LED chip comprising a radiation decoupling        surface to one surface of the carrier,    -   c) embedding the LED chip in a housing,    -   d) applying a phosphor layer to the radiation decoupling surface        of the LED chip.

It should be noted in particular that the method steps can theoreticallybe performed in any desired order; the order given here by thealphabetically arranged letters need not necessarily be followed.

In a particularly preferred embodiment of the method, in Step b) of themethod the LED chip is both mechanically attached to the carrier andelectrically contacted.

In Step d) of the described method, at least portions of the radiationdecoupling surface preferably remain uncovered. This can be achieved,for example, by covering portions of the radiation decoupling surfacewith a mask before embedding the LED chip in the housing envelope.

In a further preferred embodiment of the method, a covering body isapplied to the phosphor layer after the completion of the describedmethod. This covering body advantageously covers the phosphorcompletely. The covering body is, for example, prefabricated prior tothe application of the phosphor layer. It can then be glued to thephosphor layer. It is also, however, possible for the covering-bodymaterial to be sprayed onto the phosphor layer, shaped as desired andthen cured.

Particularly preferably, the housing in which the LED chip is embeddedis implemented as radiation-absorbing. This means that the housingmaterial is suitable for absorbing at least the bulk of theelectromagnetic radiation that is generated in the LED chip and impingeson the housing.

In a particularly preferred embodiment of the method, a multiplicity ofLED chips is attached to the carrier. Both the shape of the carrier andthe arrangement of the LED chips thereon can advantageously be adaptedto the use requirements of the arrangement.

Both the here-described arrangement and a method of fabricating such anLED arrangement are described in more detail below on the basis ofembodiment examples and the associated figures.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an LED arrangement as described herein.

FIG. 2 shows an LED chip of a kind that can be used in the describedarrangement.

FIG. 3 (a to g) depicts an embodiment example of a method as describedherein.

DETAILED DESCRIPTION

In the embodiment examples and the figures, like or like-acting elementsare provided with the same reference numerals. The illustrated elementsand the sizes of the elements with respect to one another should not beconsidered true to scale. Rather, some details of the figures have beenexaggerated in size to improve comprehension.

FIG. 1 shows the here-described LED arrangement, comprising LED chips11. Each LED chip 11 has on its top face a radiation decoupling surface12. Applied to a portion of radiation decoupling surface 12 is aphosphor layer 13.

Phosphor layer 13 advantageously contains a mixture of a luminescenceconversion material and a matrix. Said matrix is formed, for example, ofduroplastic polymers such as epoxy materials or silicone materials. Theconcentration of luminescence conversion material is advantageouslyhigh. Particularly preferably, between 30 and 50 vol. % of phosphormaterial is mixed with the matrix. Luminescence conversion materials canbe doped garnets such as YAG or TAG, chlorosilicates, alkaline-earthnitridosilicates or alkaline-earth oxynitride silicates. Suitableluminescence conversion materials are described, for example, in thedocument WO 98/12757, whose content in that regard is incorporatedherein by reference.

Phosphor layer 13 is advantageously applied to radiation decouplingsurface 12 with a doctor blade in a relatively thin between 10 and 50 μmthick, or printed on by means of special printing techniques such assilk screening or buffer printing. However, it is also possiblealternatively for the luminescence conversion material to be applied toradiation decoupling surface 12 by means of a dispensing technique, suchas for example an inkjet process, or by means of a piezo-valvedispenser.

A second portion of the radiation decoupling surface is covered by afirst contact layer 14 forming a bonding pad. Above this first contactlayer 14, LED chip 11 is electrically contacted by means of a lead 15,for example by wire bonding. First contact layer 14 advantageouslycontains aluminum and forms the n-side connection of the LED chip.

A covering body 16 is disposed on phosphor layer 13. This covering body16 preferably forms an optical element that affords the possibility ofadjusting the radiation characteristic of the LED arrangement accordingto the needs of said LED arrangement. Both refractive and diffractiveoptical elements can be used for this purpose. To this end, the sidefaces of covering body 16 preferably are substantially parabolically,hyperbolically or elliptically curved.

Since in the described LED arrangement the emission surface of phosphorlayer 13 is very similar to the chip emission surface, i.e., radiationdecoupling surface 12, the LED arrangement is particularly well suitedfor such additional optical elements. Nearly all the light emitted byLED chip 11 is converted in the phosphor layer and can then exit thearrangement through covering body 16.

Covering body 16 is advantageously formed in the same material system asthe matrix of phosphor layer 13. This has proven particularlyadvantageous during the operation of the LED chip 11, since coveringbody 16 and phosphor layer 13 then have roughly equal thermal expansioncoefficients and the connection between phosphor layer 13 and coveringbody 16 is placed under very little stress by the heat given off by theLED chip 11.

The LED chips 11 are embedded in a housing 17; hence, the side faces ofthe LED chips 11 and portions of radiation decoupling surface 12 areenveloped form-fittingly by said housing 17. The leads 15 are alsoadvantageously enveloped by the housing material. This increases themechanical stability of the connection between lead 15 and first contactlayer 14.

Particularly advantageously, the side faces of phosphor layer 13 arealso enveloped form-fittingly by housing 17. Since housing 17 ispreferably implemented as radiation-absorbing, for example black, theLED arrangement has, in the form of the surface of phosphor layer 13opposite radiation decoupling surface 12, a defined light exit surfacewith good contrast to the surrounding housing 17. Housing 17 preferablycontains a mold compound, such as duroplastic polymers (epoxy materialsor silicone materials). The mold compound is preferably implemented asradiation-absorbing. That is, the bulk of the electromagnetic radiationemitted by the LED chip and impinging on the mold compound is absorbedby the latter. In this way, the escape of radiation is deliberatelylimited to regions of radiation exit surface 12 that are covered byphosphor layer 13. The mold compound is, for example, blackened withsoot. Such a mold compound can be selected so that its thermal expansioncoefficient is well adapted to the other elements of the LEDarrangement, thus imparting high mechanical stability to thearrangement.

LED chip 11 and housing 17 are advantageously disposed on a carrier 18.Carrier 18 can, for example, be a printed circuit board (PCB), aceramic, a direct-bonded copper substrate (DBC) or a metal-core board,or it can be formed generally of structurable materials, such assilicon, for example.

Second contact layer 19 serves both to attach the LED chips mechanicallyto the carrier and to contact them electrically, for example by means ofa die-bonding process. Second contact layer 19 preferably forms thep-side connection of LED chip 11. Lead 15 is also connected to carrier18 to effect n-side contacting of the LED chip 11.

FIG. 2A is a sectional diagram of an LED chip 11 of a kind that can beused in the LED arrangement. Suitable LED chips are, for example,surface-emitting semiconductor chips based on the thin-film concept,which include a highly reflective layer 23 under radiation-generatinglayer 25. Disposed on the bottom face of the LED chip is p-side secondcontact layer 19. Second contact layer 19 is applied to a carrierelement 21.

Carrier element 21 is followed by the reflective layer 23, which isfastened to carrier element 21 by a solder layer 22. Reflective layer 23forms a mirror layer and is preferably formed of a metal. In that case,reflective layer 23 can contain, for example, silver.

The mirror layer is followed by a buffer layer 24, which for examplecontains p-doped GaN and is transparent to the radiation generated inradiation-generating layer 25. Radiation-generating layer 25advantageously includes more than one epitaxially grown layer. It isbased, for example, on a nitride composite semiconductor material, i.e.,at least one layer of the epitaxially grown layer sequence comprises amaterial from the system In_(x)Al_(y)Ga_(1-x-y)N, where 0≦x≦1, 0≦y≦1 andx+y≦1. In addition, radiation-generating layer 25 can have amulti-quantum-well structure. A single-quantum-well structure, a doubleheterostructure or a single heterostructure can also be used instead ofthe multi-quantum-well structure. In the context of this patentapplication, the term “quantum-well structure” encompasses any structurein which charge carriers undergo quantization of their energy states byconfinement. In particular, the term “quantum-well structure” implies nostatement as to the dimensionality of the quantization. It thereforeincludes, among other things, quantum wells, quantum wires and quantumdots and any combination of these structures.

Radiation-generating layer 25 is topped by cover layer 26, whichpreferably contains n-doped GaN. Cover layer 26 is preferablytransparent to the electromagnetic radiation generated inradiation-generating layer 25. It comprises on its upper surface theradiation decoupling surface 12, which is advantageously for examplerandomly structured.

This structuring of radiation decoupling surface 12 advantageouslyachieves the effect that total reflection of the radiation emitted bythe LED chip occurs less often than it would in the case of a smoothradiation decoupling surface 12. In addition, suitable structuring ofradiation decoupling surface 12 can cause the stochastic scatteringbehavior of the LED chip to be nearly ergodic.

First contact layer 14 is disposed sidewardly on radiation decouplingsurface 12. FIG. 2B is a plan view of the LED chip with radiationdecoupling surface 12 and with first contact layer 14 disposed in asideward corner. Due to this sideward arrangement of first contact layer14, first contact layer 14 takes up little surface area on radiationdecoupling surface 12 and therefore scarcely impedes the escape of theelectromagnetic radiation through radiation decoupling surface 12.

FIGS. 3A to 3G show an embodiment example of a described method offabricating the LED arrangement.

FIG. 3A is a sectional diagram of the carrier 18.

In a first method step, the LED chip 11 is mechanically attached tocarrier 18 by die bonding and electrically contacted to carrier 18 viasecond contact layer 19 (see FIG. 3B).

FIG. 3C shows the electrical contacting of the LED chip to carrier 18 bymeans of leads 15. This contacting is done by a wire-bonding process inwhich the lead 15 is attached to carrier 18 and first contact layer 14,which forms a bonding pad.

In the next method step, the LED chip is embedded in a housing 17 (seeFIG. 3D). To this end, the LED chip is enveloped form-fittingly by blackmold compound by an injection-molding process or other suitable method,such as printing processes, for example. To keep part of the radiationdecoupling surface free of mold compound so that the phosphor layer 13can subsequently be applied to it, a mask can be used that keeps theselocations on radiation decoupling surface 12 bare. Alternatively, theportions of radiation decoupling surface 12 to which phosphor layer 13is to be applied can be re-exposed by etching after molding. It isfurther possible for phosphor layer 13 to be to be applied to radiationdecoupling surface 12 of the LED chip even before overmolding. Inparticular, phosphor layer 13 can be applied to radiation decouplingsurface 12 as early as the wafer composite stage.

During the overmolding of the LED chip 11, the upper surface of thecarrier not covered by the LED chip is also advantageously envelopedform-fittingly by molding compound. The first contact layers 14 and theleads 15 are also preferably overmolded as well. This results inmechanical stabilization of the LED arrangement. The molding compoundcan also advantageously be admixed with fillers (for example adhesionmediators or CTE) for good adaptation of the housing to the carrier. Forexample, this can be done to adapt the thermal expansion coefficient ofthe housing to that of the carrier. This measure advantageouslyincreases the mechanical stability of the arrangement.

FIG. 3E shows the application of phosphor layer 13 to radiationdecoupling surface 12. The side faces of phosphor layer 13 are alsoadvantageously enveloped by the housing material, so thatelectromagnetic radiation generated in radiation-generating layer 25exits for the most part through the surface of phosphor layer 13 that isopposite radiation decoupling surface 12.

FIG. 3F illustrates the application of the covering body 16 to phosphorlayer 13. This covering body 16 can, for example, be composed ofprefabricated optical elements, which are glued onto phosphor layer 13.However, it is also possible, for example, for covering body 16 to beprinted onto the not-yet-cured phosphor layer 13 and form a permanentmechanical connection with phosphor layer 13 once it has cured.

Alternatively, covering body 16 can be sprayed onto phosphor layer 13.In this case, use can be made of thixotropic materials that are sprayedonto phosphor layer 13 and are then given the desired shape. In thiscase the covering body preferably contains silicone. It is also possibleto apply high-surface-tension materials to phosphor layer 13 in dropletform and then cure them, optionally using ultraviolet radiation.

In a further method step (see FIG. 3G), the LED arrangement can besingulated so as to yield LED arrangements possessing the desired shapeand number of LED chips 11. Alternatively, the LED arrangement can, ofcourse, be fabricated in the desired shape and with the desired numberand arrangement of LED chips 11 right from the start.

This patent application claims the priority of German Patent Application102004021233.3-33, whose disclosure content is incorporated herein byreference.

The invention is not limited by the description based on the embodimentexamples. Rather, invention encompasses any novel feature and anycombination of features, including in particular any combination offeatures recited in the claims, even if said feature or said combinationitself is not mentioned explicitly in the claims or the embodimentexamples.

1. A method of fabricating an LED arrangement, comprising the followingsteps in the following order: a) preparing a carrier, b) attaching tosaid carrier an LED chip comprising a radiation decoupling surface, c)embedding said LED chip in a housing, such that said LED chip isenveloped by said housing with said housing in contact with andpartially covering the radiation decoupling surface of said LED chip, d)applying a phosphor layer to said decoupling surface of the LED chip. 2.The method of fabricating an LED arrangement as set forth in claim 1,wherein a covering body is disposed on said phosphor layer.
 3. Themethod of fabricating an LED arrangement as set forth in claim 1,wherein the housing is configured to absorb radiation generated in theLED chip that impinges on the housing.
 4. The method of fabricating anLED arrangement as set forth in claim 1, wherein a multiplicity of LEDchips is attached to said carrier.
 5. The method of fabricating an LEDarrangement as set forth in claim 1, wherein said LED chip is envelopedby a mold compound.
 6. The method of fabricating an LED arrangement asset forth in claim 5, wherein said LED chip is enveloped by aninjection-molding process.
 7. The method of fabricating an LEDarrangement as set forth in claim 5, wherein said LED chip is envelopedby a printing process.
 8. The method of fabricating an LED arrangementas set forth in claim 1, wherein a covering body is arranged on saidphosphor layer.
 9. The method of fabricating an LED arrangement as setforth in claim 8, wherein the covering body is implemented as an opticalelement.
 10. The method of fabricating an LED arrangement as set forthin claim 8, wherein the phosphor layer contains a matrix material intowhich a luminescence conversion material is mixed and the covering bodycontains the same matrix material.
 11. The method of fabricating an LEDarrangement as set forth in claim 8, wherein the covering body iscomposed of a prefabricated optical element which is glued onto saidphosphor layer.
 12. The method of fabricating an LED arrangement as setforth in claim 8, wherein the covering body is pushed into thenot-yet-cured phosphor layer and a permanent mechanical connectionbetween said phosphor layer and the covering body is formed by curing ofsaid phosphor layer.
 13. The method of fabricating an LED arrangement asset forth in claim 1, wherein said LED chip is a thin-film LED chip. 14.A method of fabricating an LED arrangement, comprising the followingsteps in the following order: a) preparing a carrier, b) attaching tosaid carrier an LED chip comprising a radiation decoupling surface, c)embedding said LED chip in a housing, d) applying a phosphor layer tosaid decoupling surface of the LED chip; wherein a covering body isarranged on said phosphor layer, the covering body composed of aprefabricated optical element which is glued onto said phosphor layer.15. The method of fabricating an LED arrangement as set forth in claim14, wherein the housing is configured to absorb radiation generated inthe LED chip that impinges on the housing.
 16. The method of fabricatingan LED arrangement as set forth in claim 14, wherein a multiplicity ofLED chips is attached to said carrier.
 17. The method of fabricating anLED arrangement as set forth in claim 14, wherein said LED chip isenveloped by a mold compound.
 18. The method of fabricating an LEDarrangement as set forth in claim 17, wherein said LED chip is envelopedby an injection-molding process.
 19. The method of fabricating an LEDarrangement as set forth in claim 17, wherein said LED chip is envelopedby a printing process.
 20. The method of fabricating an LED arrangementas set forth in claim 14, wherein said LED chip is a thin-film LED chip.